A known shield-type tunnel excavator comprises a digging tool mounted on a shield which is advanced as excavation proceeds in a forward digging chamber which is sealed by a separating wall of the excavator shield from the tunnel already formed behind the shield.
The tunnel is usually not under an elevated pressure.
The separating wall is of the transverse cross section of the excavator and the digging chamber is filled with a fluid supporting the forward face at which digging is effected. The drive of the excavating tool and the removal of the excavated material is effected through this separating wall.
Various forms of shielded tunnel excavators are known as described, for example, in German Patent DE-PS No. 27 60 000, German Open Patent Application DE-OS No. 27 09 739 and German Printed Patent Application DE-AS No. 27 05 580. They are used in soft, water-bearing earth in which the tunnel should not be under excessive pressure and the soft local forward wall of the digging chamber is sustained with a pressurized viscous liquid, forming the supporting fluid.
Thus it is important to keep the volume excavated in correct relation to the excavation speed and to the transport rate of excavated material, because the supporting fluid is incompressible.
Especially when too much material is excavated and removed in relation to the tunneling distance the danger of settling exists, because the pressure of the supporting liquid may drop.
With these tunnel excavators one tries to maintain the pressure of the supporting fluid in the digging chamber constant with the help of expensive control mechanisms. However, the goal of these control mechanisms is attained only very incompletely. When the viscosity of the supporting fluid is high it is particularly difficult to keep the pressure constant since a measurement valid for the entire cross section is not possible and the filling and/or amount of the supporting fluid is subject to continual fluctuations in the earth removal required by the excavation.